The invention relates to drill bits and methods of manufacturing drill bits, and is particularly applicable to drag type bits. More specifically, the present invention relates to drill bits formed by powder metallurgy wherein the cutter assemblies are brazed into substantially enclosed, dimensionally controlled pockets formed in the bit matrix.
The use of drag type drill bits in the drilling of wells, particularly oil and gas wells, is well known. The typical drag type drill bit serves to transfer the weight of the drill string to the bottom of the borehole. The protruding cutters on the bottom surface of the drill bit serve to cut into the formation as the drill bit rotates. In extremely hard formations, the cutters essentially crush or fracture the formation by means of the large compressive force applied to the formation. In such hard formations, the cutters may take form of relatively small diamonds, e.g., 1/10 karat. In soft formations, the drag type drill bit essentially plows through the formation. The typical drill bit used in relatively soft formations may utilize large cutters, e.g., 2 karat.
Numerous considerations must be taken into account in the design of a drag type drill bit. As mentioned above, the cutter size is typically dictated by the hardness of the formation. Furthermore, the cutter may be chosen to have a specific profile, e.g., round or V-shaped. The concentration and placement of the diamonds along the face of the drill bit are critical to drill performance.
Careful attention must be given to the hydraulic characteristics of the bit since the drill bit must remain relatively cool and the cuttings must be promptly swept away from the drilling interface so that they are not reground. To this end, intricately designed waterways or fluid courses are provided on the face of the drill bit to direct the drilling mud across the face of the drill bit to effect cooling and cleaning. Typical hydraulic designs provide for either radial flow or circumferential flow, or some combination thereof. The hydraulics of the bit are also affected by the "profile" of the bit, i.e., the radius of curvature at the face of the bit.
Commercial drag type drill bits presently in use are typically formed by powder metallurgy techniques wherein a graphite mold is made to the shape of the bit. Depressions are carefully located in the mold and natural diamond cutters are glued into place in the depressions. A tungsten carbide powder is placed into the mold and infiltrated in a furnace cycle with a copper alloy with a steel shank in place. The maximum temperature in the furnace cycle may be on the order of 2200.degree. F. The bit is allowed to drop out of the mold and is finished by welding to the steel shank an extension including the pin, and final machining.
Recent developments in the manufacture of drag type drill bits suggest that synthetic polycrystalline diamond drill blanks may be utilized as the cutters in such bits. These synthetic cutters have the unique advantage of being uniformly shaped, as opposed to the varying shapes of natural diamonds. However, present synthetic diamond drill blanks cannot be placed in the matrix prior to furnacing as can natural diamonds because the synthetic diamond cannot withstand temperatures on the order of 2200.degree. F.
Synthetic polycrystalline diamonds in a disc form have been brazed directly onto the matrix of drag drill bits for use in soft formations. To date, the synthetic diamond discs which have been secured to drag type drill bits have been relatively large, e.g., 1/2 inch in diameter, because of limitations in the reliable attachment of the cutter to the drill bit matrix. Because the cutters are not located in a single plane, some form of positive fixturing must be used when attaching the cutters to the previously formed and furnaced drill bit head.
It has been proposed to use dead weights and various camming arrangements to fixture synthetic diamond cutters during brazing operations. However, such fixturing techniques have proven extremely complex and unreliable. It also has been suggested to fixture the synthetic diamond cutters by the use of shims of a high expanding metal that expand during the heating of the brazing operation and are removable upon cooling. This approach has also proved disadvantageous. Thus, it can be seen that there is an acute need for a drill bit having securely attached cutters, for example, synthetic polycrystalline diamond cutters, attached after furnacing of the bit head matrix. Improved attachment methods make possible the use of relatively small cutters for use in medium and hard formations.
Several methods for tangentially attaching cutters to the drill bit using brazing techniques have been proposed. In methods presently used, it has been found that simple metallurgical brazing normally results in inadequate securement of the cutter in the cutter pocket. Presently in the art, brazing foil is circumferentially wrapped around a drill blank or cutter and fit into the cutter pocket in the drill bit head. The brazing foil is normally 0.003 inches in thickness. The industry standard is wrapping at least three times around the circumference of the cutter to achieve any brazing at all. Thus, the tolerance between the cutter and the cutter pocket must be at least three times the thickness of the brazing foil or 0.009 of an inch per side. In order to fit into the cutter pocket, the tolerance must be dimensionally greater than 0.009 of an inch. It is known that upon furnacing the brazing material approximately 40% of the volume of the material is lost. The tolerances are great in order to achieve the wrapping of the foil and fitting into the pocket on the bit head. A "puddling" results with the brazing material collecting at the bottom of the pocket and attaching the cutter to the pocket only at that point.
For use of the Stratapax.TM. cutters manufactured by General Electric, the standard tolerances recommended are 0.009 inches per side for the brazing foil with three wraps plus 0.006 inches for a total of 0.015 inches per side between the cutter and the pocket.
Further, since the brazing techniques are mainly directed to a puddling affect where attachment is at the bottom of the cutter pocket, the cutter pocket itself is enclosed to less than 60% in commercial drill bits.
It has been found that the production cutter must be able to withstand a high degree of mechanical load. Because of this mechanical load and the inadequacy of the presently used brazing techniques, the cutters are found to be breaking off during use. Another problem found with the present cutter attachment methods is in the use of drilling fluid to clean the cuttings and cool the bit head. When high velocity fluids are passed over the drill bit head, the brazing material is eroded due in part to the gaps between the cutter and the cutter pocket.